1. Field of the Invention
The invention relates to a controlled temperature reactive sintering process for producing finely divided intermetallic and ceramic powders, particularly nickel aluminide powders, to the use of these powders as binders for cutting tools and to compositions and processes for injection molding using these powders.
2. Information Disclosure
Intermetallic compounds are current candidates for use as turbine blades, engine components, dental and surgical instruments, heating elements, and several other applications requiring high temperature, oxidation-resistant materials. The intermetallic compounds based on aluminum (e.g., nickel aluminide, titanium aluminide, iron aluminide, and niobium aluminide) have the attractive characteristics of low density, high strength, good corrosion and oxidation resistance, and relatively low cost. In some cases, the intermetallics exhibit the unique property of increasing strength with increasing temperature. This property coupled with relatively high melting temperatures make for ideal high temperature materials. The specific combination of low density and high strength (referred to as a high strength-to-weight ratio) makes these materials excellent candidates for uses in which high strength is required in conjunction with minimum weight.
Reactive sintering is a powder metallurgy process which can be used to create intermetallic and ceramic compounds. The reaction is sustained by a transient liquid phase generated by the exothermic self-heating associated with compound formation. Reactive sintering is a special case of combustion synthesis in which densification occurs in conjunction with the combustion synthesis process. The transient liquid phase that is generated aids in the densification process. Processing time is on the order of one hour to produce high-density and high-strength parts from mixed elemental powders. Heat is liberated in the process as the constituent powders react to form an intermetallic compound and the reaction is thus self-sustaining. The process has many variants and names including reactive sintering, self-propagating high temperature synthesis (SHS), and combustion synthesis. Compound systems being developed with the process range from intermetallics such as NiAl, TiAl, MoSi.sub.2, Ni.sub.3 Si, Ni.sub.3 Al, Ni.sub.3 Fe and NbAl.sub.3 to ceramics such as TiC, TiB.sub.2, Si.sub.3 N.sub.4, NbN and WC. SHS techniques are attractive because they involve low processing costs and produce intermetallic compounds at relatively low temperatures.
U.S. Pat. No. 4,762,558 (German et al.) relates to the formation of a densified Ni.sub.3 Al compound employing reactive sintering on a shaped compact. The patent discloses a means for forming nickel aluminide intermetallic shapes by reactively sintering a compacted mixture of elemental nickel and aluminum powders to form a dense structure. By this approach densified parts and shapes may be formed from the elemental powders. The process of the '558 patent is well suited to the formation of monolithic, uniformly highly dense bodies of Ni.sub.3 Al. For many manufacturing applications however, it would be highly desirable to have very finely divided powders of intermetallics. Unfortunately the very properties of intermetallics, in this case nickel aluminide, that make them attractive also make them difficult to comminute. Dense, monolithic bodies produced by methods similar to U.S. Pat. No. 4,762,558 are not easily comminuted into powders.
For this reason intermetallic powders are typically produced by an atomization process in which a stream of molten metal is broken up into droplets by a stream of liquid, in most cases water, or by a jet of gas. The droplets then solidify to form metal powders. Intermetallics pose a special problem for atomizing because of the tendency of the material to oxidize at the high temperatures required for processing. Additionally, it is difficult to form the proper intermetallic compound because of segregation of the elemental species (i.e., nickel and aluminum for Ni.sub.3 Al) during solidification. The particle sizes which are formed are not sufficiently fine in diameter (i.e., 20 micrometers and less) for applications requiring lower sintering temperatures and for processes such as powder injection molding. Intermetallic powders, which are formed currently by atomizing, are in short supply and are very costly.
Clearly, a need exists for processes for producing powders that permit the use of commercially available starting materials, comparatively low processing temperatures, and short process times.